Method and apparatus for using a photoacoustic effect for controlling various processes utilizing laser and ion beams, and the like

ABSTRACT

A novel control method and attendant apparatus for controlling other various methods/processes by utilizing a photoacoustic effect principle, whether or not the other various processes are with regard to solid or non-solid fluid media, but which other various processes use and require control of a primary excitation high energy beam and related beam-modulating and monitoring apparatus for establishing and detecting a photoacoustic effect. This method and apparatus require the use of appropriate beam generating and beam-generating power control means for controlling such an energy beam, the latter of which may be of the type including laser beams, microwave beams, x-rays and other electromagnetic wave beams, as well as ion, electron, particle, and molecular beams. Various beam modulating means may be used as well as various techniques for monitoring and detecting the photoacoustic effect, such techniques including different apparatus related to respectively different techniques, including a gas cell means, mirage generating means, piezoelectric detector means, infrared radiation detector means, and the reflective probe beam means.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without payment to meof any royalty thereon.

BACKGROUND OF THE INVENTION

The present invention is a continuation in-part application of mycopending U.S. patent application Ser. No. 496,366 filed May 20, 1983,now U.S. Pat. No. 4,476,150, and relates broadly to utilizing aphotoacoustic effect-producing process to generate or return informationuseful in turn to effectively control other processes.

More particularly, the invention relates to utilizing variousphotoacoustic type modes or detection techniques for ultimatelycontrolling certain processes by means of the photoacoustic modedetection of certain characteristic temperature variations in aprocessing area undergoing processing treatment by a primary excitationenergy beam. The term "excitation" as used herein is to indicate thatthe selected type of beam is excited in a manner so as to establish aphotoacoustic effect. Thus by such photoacoustic effect usage to monitorany chosen primary excitation beam, in conjunction with certain variousbeam modulating means, with various suitable transducer means togenerate certain electrical signals, and together with ancillaryelectrical control circuit means, various types of processes can beeffectively controlled by this unique control of the primary excitationbeam. The range of specific processes contemplated to be controlledinclude not only laser beam annealing of thin-film-like ceramic materiallayers which have been applied to substrates by chemical vapordeposition (CVD), of the type described in my above-noted copendingpatent application, but also various other processes such as laserscribing and machining, laser-assisted chemical vapor deposition (LCVD),ion implantation to impart improved wear and fatigue-resistanceproperties, and corrosion resistance and the like, and production ofpowders via the chemical vapor deposition (CVD) process, among others.

The types of primary excitation beams may include laser beams, microwavebeams, x-ray and other electromagnetic wave beams, as well as ion,electron, particle and molecular beams.

It is to be understood that the photoacoustic effect referred to andbeing primarily described herein may be considered in conjunction withphotoacoustic microscopy. In this regard, the present primary interestlies in detecting and monitoring response differences of small localizedspots or areas primarily on solids which are being periodically butrelatively quickly heated by modulated primary excitation beams. Thatis, the main interest lies in detecting certain characteristic periodictemperature variations of the monitored spots or areas, whether withregard to a solid or non-solid fluid medium undergoing beam-relatedprocessing; and wherein the aforesaid temperature variations maymanifest themselves in various ways such as pressure, index ofrefraction, thermoelastic, or infrared radiation emission changes. Thus,control of processes relating to the processing of gaseous mediums isalso contemplated, such as monitoring a gas stream in a chemicalmanufacturing plant. With regard to the term photoacoustic as being usedherein, the `acoustic` portion may appear to be a misnomer. This isbecause the monitoring or detecting is relative to periodic temperaturevariations and mostly without regard to any associated true `noise` or`acoustical` characteristic, except as to that attendant to the formdepicted in FIG. 2. In the latter form of FIG. 2, which utilizes aphotoacoustic gas cell means the detected pressure variations areconsidered to be sounds in the air. Thus, as recognized by researchscientist, Allan Rosencwaig, who is one of the hereinafter-mentionedreferenced authors of some relevant work, the term `photoacoustic` couldbe more aptly named `photo-calorimetry`. Additional more apt terms mayinclude ion-calorimetry, thermal wave imaging, and the like.

By comparison to photoacoustic microscopy, in the somewhat allied fieldof photoacoustic spectroscopy of solids, the main interest is in theresponse differences of the solid as compared to the light frequencychanges, while still using a modulated excitation beam. In either form,the modulated excitation beam, as stated hereinabove, can be any form ofelectromagnetic energy, such as radio frequency waves, microwaves,infrared waves, visible rays, ultraviolet rays, x-rays, and gamma rays.Particle type beams which may be used include electron, proton, neutron,ion, atomic, and molecular.

While it is recognized that many studies have been previously conductedon laser interactions with various metals, a significant amount of thesehave been in the non-analogous sub-categories of semiconductors andintegrated circuitry used in the non-analogous per se broader field ofelectronic communications.

It is also recognized that there are recent and ongoing studies in thesefields of Photoacoustic Microscopy, Photoacoustic Spectroscopy, andElectron-Acoustic Microscopy as evidenced by the following recentpublications:

1. "What Is Photoacoustic Spectroscopy" By J. A. Noonan and D. M.Munroe; Published in "Optical Spectra", the International Journal ofOptical/Electro-Optical/Laser Technology February 1979.

2. "Photoacoustic Microscopy of An Integrated Circuit" by L. D. Favro,P. K. Kuo, J. J. Pouch, and R. L. Thomas, Department of Physics, WayneState University Detroit, Mich. 48202; Published in the "Applied PhysicsLetters 36 (12), 15 June 1980, by the American Institute of Physics.

3. "Electron-Acoustic Microscopy" by G. Slade Cargill I11; Published inPhysics Today, October 1981.

4. "Photoacoustics and Photoacoustic Spectroscopy" by Allan Rosencwaig,pg. 296, 297. Published by John Wiley & Sons, New York, N.Y. (1980).

Notwithstanding a statement on pg. 297 in the latter-mentioned articlethat photoacoustic microscopy appears to hold considerable promise bothas general analytical tool and as a dedicated processcontrol instrument,that author is speaking specifically with regard to the semiconductorindustry, and has not expanded his discussion therein to contemplateutilizing the photoacoustic effect for controlling other processes orprocess-controlled systems of the type being described hereinafter.Furthermore, it is believed that research personnel working in thefields of laser and ion beam processing do not normally or regularlyoverlap with research personnel working in the field of photoacousticsand the like. This is considered so notwithstanding that authorRosencwaig, among others, recognizes or acknowledges that photoacousticsignals can be generated through thermal excitations arising from theinteractions with a sample of particle beams, such as beams ofelectrons, protons, neutrons, ions, atoms, or molecules; and that suchsignals also can be generated through the absorption of any and allforms of electromagnetic energy such as radiofrequency waves,microwaves, infrared, visible, and ultraviolet light, x-rays andgamma(γ) -rays.

Proceeding herewith, reference is again made to my copending applicationSer. No. 496,366 filed May 20, 1983, incorporated herein by referenceand primarily directed to laser beam annealing relieving of residualstresses for layers of CVD silicon nitride and CVD silicon carbide knownrespectively as CVDSN and CVDSC. My copending referenced applicationalso relates generally to the method of monitoring and controlling thislaser beam annealing or treating process by a technique known asphotoacoustic microscopy. As stated therein, this basic techniqueinvolves the use of a laser type visible light beam to produce localizedperiodic heating of a sample surface. In the example given thereininvolving an acoustic gas cell, the heating of the trapped gas withinthe cell also produced an acoustic signal. In photoacoustic microscopy,while the energy beam is concentrated on a localized spot, the spot'sindividual characteristics can be and are probed or monitored.Additionally, an image can be formed on and with certain associatableequipment, from the localized responses of adjacent spots, for exampleof an integrated circuit.

Although some of the copending application content will be repeatedherein, it is to be understood that all or various parts thereof areequally considered to be incorporated herein and/or may be selectivelyincorporated herein by persons desiring it or finding the need thereofto more completely fulfill their understanding of this application.

Thus, the principal objective of the present invention includesutilizing the photoacoustic effect and variousphotoacoustic-effect-producing means to monitor and control the variousaforementioned processes while those processes are being performed, ascompared to utilizing the effect or process to evaluate the results atthe completion of the process.

This and similar processes and the contemplated control thereof will bedescribed in further detail hereinafter, taken in conjunction with thefollowing illustrative drawing figures.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a generally diagrammatic perspective view of a movably mountedCVD ceramic coated workpiece being subjected to a laser beam annealingprocess to relieve the residual stresses;

FIG. 2 is a perspective depiction of a photoacoustic effectmonitoring/controlling process with the aid of at leastsemi-diagrammatically illustrated apparatus for effecting the monitoringby one form of photoacoustic effect generated by a modulated laser beam;

FIG. 3 is a diagrammatic representation of means producing aphotoacoustic effect utilizing an Infrared (IR) detector means formonitoring and controlling a laser beam processing;

FIG. 4 is a similar diagrammatic representation of means producing aphotoacoustic effect but utilizing a so-called "MIRAGE" technique, whichis related to changes in the index of refraction, for monitoring theheated spot and the attendant control of the excitation beam thereof;

FIG. 5 is a similar diagrammatic representation, which utilizes apiezoelectric technique; and

FIG. 6 is a still further diagrammatic representation, which utilizes aso-called "Reflection" technique wherein an associated laser probe beamis reflected off the so-called "hot spot" of the heated area.

As a matter of further background concerning more particularly the firstto be discussed embodiments (FIGS. 1 & 2), with respect to processesinvolving chemical vapor deposition (CVD), two gases are intermixed sothat the reaction product is deposited upon adjacent surfaces of a workpiece substrate member. For example, silicon tetrachloride and ammoniain gas form can be mixed to react so as to form silicon nitride,sometimes denoted CVDSN. The silicon nitride coatings thus formed can beof almost theoretical density, of high purity, and are also very hard.When a laser is used to assist this process it is often known as LCVD.In the photolytic route, the laser frequency is chosen so that itexcites the gases which then react, or else the laser light frequencyexcites another chemical which then causes the desired reaction. In thepyrolytic or heating route, the laser is used to heat the gases whichthen react, or alternatively, the laser heats the surface on which thereaction product is to be deposited and the gases react at the heatedsurface spot. Advantages of the LCVD include spatial resolution andcontrol, cleanliness, rapid deposition rates, minimization of substratedistortion and rapid heating and cooling rates.

Regarding the photoacoustic effect, as applied to solids in the mostcommon way, the effect begins with a modulated light beam incident on asolid. Part of the light is absorbed and through radiationlesstransitions is converted into heat in the solid. The heated surface ofthe solid then heats the gas in contact with it. If or where the solidheated surface is enclosed within a cell with a transparent window toallow the light to pass therethrough, this heated gas causes a pressureincrease within the so-called gas cell. Then, because the light beam ismodulated, the gas pressure variations are modulated at the samefrequency, i.e., sound is produced. A small microphone, such as the typeused in hearing aids, can be and often is used as the detector of thissound, and serves as the transducer means for converting the sound intoan electrical signal. The photoacoustic effect is dependent on theabsorption of the radiation and subsequent heating of the surface onwhich the beam radiation is incident. Thus, the technique is sensitiveto both optical absorption and thermal properties which includes thermalboundary conditions. This sensitivity to thermal properties allows thetechnique to probe beneath optically opaque surfaces. An enclosed gascell of this type is more commonly known as a photoacoustic cell, andrepresents but one technique or manifestation called photoacousticmicroscopy for utilizing the photoacoustic effect to monitor and effectbeam control of the aforesaid types of processes such as CVD and LCVD,or others.

Other photoacoustic microscopy process modes using different equipmentare contemplated and now will be discussed in more detail. Briefly, useof an enclosed gas cell technique is not necessary when using thedifferent so-called "Mirage" detection technique. In this "Mirage" form,the index of refraction change of the heated gas (above the surface) isused to deflect a second generally transversely disposed laser beamcalled a probe beam. The probe beam is not for the purpose of excitinganother photoacoustic effect, but only for monitoring and detecting thebasic photoacoustic effect. Thus, as the gas is heated more or less, theprobe beam is correspondingly deflected more or less, and in conjunctionwith suitable optic transducer detector means generates certain signalsuseful in conjunction with ancillary signal processing means to controlthe beam strength or power.

A further alternative technique is use of a piezoelectric sensor whichcan be mounted upon the solid to detect the thermoelastic response ofthe solid. This latter approach allows the use of a modulated electronor ion beam for the energy source, which then requires the solid andbeam to be housed within a vacuum.

These other modes/techniques including a "Reflection" technique will befurther described in conjunction with additional subsequent drawingfigures representative of those techniques.

Referring first to FIG. 1, a laser beam generator means 10 is suitablysupported in association with an appropriate laser power supply controlmeans 12, which together with the generator means 10 effectivelygenerate a suitable laser beam 14 and include means to vary the powerand power density of the laser as is known in the laser control art. Aworkpiece or substrate 16, having a CVD coating 18 is suitably removablyaffixed onto a portable support assembly 20 capable of effecting travelof the ceramic-coated workpiece 16 so as to selectively traverse beforethe convergingly focused laser beam portion 14a. Suitable lens means 22is used to focus the beam 14 if higher power density is required than ispresent in the original beam. The lens size, type, and focal length orpower will depend upon the particular application undergoing treatment,including whether the coated part to be treated is of planar or arcuateor other irregular form.

Support assembly 20 may be of the motorized type with dual platforms24,26 each adapted to be moved in various directions to provide controlof the workpiece in three orthogonal planes or directions x--x, y--y andz--z. This mechanism or assembly is only shown schematically because itis well known in the art and does not constitute a point of novelty inthis invention.

In operation, an exemplary laser such as a CW Argon Ion laser isenergized to produce the beam 14. Beam focused portion 14a terminates ina small size dot or spot area, the ultimate size of which will determinein part the laser beam power density of between approximately 1-10KW/cm², so as to generate a surface temperature of between about1000°-1600° C. for exposure times ranging between approximately 10-200milliseconds. From this treatment, favorable reordering of the CVDcoating is expected to occur so as to effectively relieve the inherentresidual stresses found therein. The relieving of these stresses isfurther described in my aforesaid copending application Ser. No. 496,366filed May 20, 1983.

FIG. 2 represents an exemplary system and process for practicing themonitoring and control of a laser annealing process to relieve inherentresidual stresses in a ceramic coated component. In this FIG. 2 example,it is understood that a suitable laser beam source means comparable tothat of 10,12 in FIG. 1 is to be utilized in this embodiment, to producea high energy laser beam 14'. Depending upon the arrangement of thesystem's various components, it may be necessary to use one or moremirrors to achieve the desired direction of the beam. In this example, amirror 28 is angularly disposed to reflect beam 14' downward toward theworkpiece 16'. A suitable lens 22' is appropriately supported in aposition to effect focusing of the laser beam to a point P upon theworkpiece 16'. Workpiece 16' is similarly coated with a layer 18' ofCVDSN or CVDSC, and is to be anneal treated to relieve it of undesirableresidual stresses.

A suitable photoacoustic cell assembly 30 is placed upon the coatedsurface 18' of the workpiece. Photoacoustic cells, primarily intendedfor gas or small solid or liquid samples, are commercially availablefrom such companies as Photo-Acoustics of Rochester, Mich.; EG&GPrinceton Applied Research Corporation in Princeton, N.J.; and Rofin,Inc. of Newton Upper Falls, Mass.

The cell to be described herein is to be more adaptable to surfaces oflarger parts, although it utilizes the same principles. In thisexemplary form, the body 32 is provided with an open bottom pocket orcell 34 convered at the upperside by a sealed transparent window 36.Adjacent to the cell pocket 34 is a smaller pocket or more shallow cell38 open face up to the body 32, but suitably covered by a miniaturemicrophone 40. The smaller microphone cell 38 is in gaseous andacoustical communication via a lateral shallow connecting passageway 42.Microphone wiring 44 connects with a suitable terminal connecting block46 which preferably is attached to the main cell's body 32. Detachableconductor wire cable 48a, in turn, connects the microphone 38 first witha suitable microphone preamplifier means 50, which may be powered byswitch-controlled 11/2 volt battery contained therein, and then via asecond conductor wire cable 48b with an appropriate insignal connectorreceptacle 52 on a lock-in amplifier means 54. Thus, it is apparent thatthe noises or signals generated within the photoacoustic cell areamplified and transmitted as input signals to the lock-in amplifier 54.An example of a commercially available lock-in amplifier is the Model186A Synchro-Het, manufactured by Princeton Applied Research Corporationin Princeton, N.J. It is also understood that the microphonepreamplifier means 50 may be of different design configuration and maybe integrated with the photoacoustic cell as is the case with somecommercially available cells.

To facilitate good results with these cells, the perimeter of the cellarea should be in good, close contact with the coating surface. This canbe achieved in various ways, one of which is to have the cell perimeter,at the underside thereof, coated with a thin layer of a suitableflexible sealing media, such as wax or wax-like substance. The cellvolume should also be minimized to thereby reduce the volume of gasbeing pushed against or heated, and thus provide a better signal tonoise ratio.

To further meet the prerequisites for establishing the photoacousticeffect, i.e., that the laser beam becomes a modulated beam while focusedon a work surface spot to heat the spot beneath the photoacoustic cellwindow, as well as means for continuously detecting the progressivelyheated spots as the work piece is moved relative to the focused beamspot, the exemplary beam-modulating means designated generally by thereference character 56, will now be described.

The illustrated beam-modulating means 56, is to be consideredrepresentative of such known devices. One commercially available deviceis known as Series 7510 Optical Micro Chopper, manufactured by RofinInc., Newton Upper Falls, Mass. 02164.

Beam-modulating means 56 comprises a rotary slotted disc means 58 whichincludes conventional means for facilitating its removable attachment onthe drive shaft 60 of the small electrical drive motor 62. Discs havinga greater number of smaller size slots may be interchangeably used ifdesired. Motor 62 is suitably mounted upon an arm 64 of a supportbracket 66 disposed to enable the focused beam portion 14" to passthrough the slotted portions of the rotary modulating disc 58. Thus, itis apparent that during operation, the beam 14" is mechanically choppedor interrupted repeatedly to effect its modulation. Alternatively, themodulating means may chop or modulate beam 14" so that the disc 58 isnot exposed to excessive laser power densities. The beam is preferablychopped in the audio frequency regime of approximately between 1 H_(z)and 20 KH_(z). The device further includes optical emitter-detectormeans disposed on opposite sides of the disc in a bifurcated portion 68of the bracket 66. The emitter-detector means may include a small lampor a light emitting diode (LED) 70 as the light source and an opposedsmall detector diode 72 such as a silicon photo detector, or any similartype detector which responds to a light source so as to detect thechopped or interrupted light source 70. Such means may alternativelyinclude small Infrared (IR) wave emitter-detector means. Onecommercially available form of optical emitter-detector sensor means iscalled an IR Sensor Limit Switch manufactured by General InstrumentCompany of Palo Alto, Calif. 94304.

Thus, the emitter-detector means 70,72 are appropriately wired to formpart of the circuit means which effects conversion of the rate at whichthe laser beam is chopped into an appropriate electrical signal with atleast part of the circuit means being within the bracketmounted boxhousing 74. Housing 74 is provided with an appropriate terminalreceptacle to receive one connector end portion 76 of a flexible coaxialconductor cable 78, of which the opposite end connector portion 80 isadapted to be received in reference signal connector receptacle 82 ofthe lock-in amplifier 54. Therefore, the latter described means 70-80establish an electronic reference signal input to the lock-in amplifier,in a parallel manner to the photoacoustic cell's pressure changemodulated signal input at the aforesaid other receptacle 52, at theopposite side of said amplifier. The two distinct input signals into thelock-in amplifier 54 are processed internally in a known manner toprovide a particular D.C. voltage, the voltage signal thereby beingvisually discernable on the voltage meter 84 of the amplifier 54. Theproduced D.C. voltage is proportionate to that part of the input signalthat is at the same frequency as the reference signal and at a set orchosen phase angle with respect to the reference signal. It isunderstood by those familiar with the art that the photoacoustic effecthas both a magnitude and phase value with respect to the beammodulation.

The resultant signal is then transmitted out via conductor cable 84 asan output signal to an appropriate feed back circuit means 86 which mayinclude microprocessor and/or other computer means in conjunctiontherewith, and then via a further conductor cable 88 to a laser powersupply means (such as 12 in FIG. 1) to effectively and essentiallyinstanteously modify the laser beam strength or wave amplitude asconditions and desired mode may dictate. Use of lock-in amplifiers andrelated control circuit means are well known to electrical controlengineers working in the art. It is understood that if a pulse typelaser is used, then the control would be directed to controlling thepulse power, the pulse width, or within limits, the repetition rate ofthe pulsing means associated therewith. Thus, when the laser heated spotwithin the gas cell is discerned by the photoacoustical monitoring asbecoming too hot, thereby endangering burning through the CVD layer orimparting any other adverse treatment effect, the microprocessor andfeed back circuitry means are used to effectively control the laserbeam's effective strength for the required time, including anythereafter increase or decrease or maintaining of the desired level ofpower as required during and throughout the laser treating or annealingprocess. This composite process is expected to favorably relieve thedetrimental residual stresses.

One contemplated use of this novel process will be more particularlyapplicable to monitoring and more effectively controlling the laser beamduring laser treatment on CVD ceramic-coated substrates involvinginhomogeneous characteristics such as where the substrate requires orotherwise has variations in layer thickness or density, thus resultingin non-uniform optical and thermal absorption properties. As a result,such conditions would inherently require close monitoring and accuratecontrol of the laser beam intensity.

Discussion will now relate to the utilization of some other perhapslesser known monitoring and modulating techniques which I contemplateutilizing in conjunction with or as part of the photoacoustic effect fordetecting or monitoring of excessive hot spots of a modulated energysource beam. The selected beam may be a laser beam or an ion beam, orother similar high energy beam or light source. As already brieflymentioned earlier herein, these other modes include use of an infrared(IR) detector technique, the so-called "Mirage" technique, thepiezoelectric detector technique, and the "reflection" technique.

An exemplary arrangement using an IR detector technique is illustratedin FIG. 3. As shown, an intense light beam such as a focused laser beam14c, suitably modulated as by a rotary chopper disc 58c or other aptmeans, is directed upon the coated substrate or workpiece 16c at pointP'. The modulated beam generates a desired heated area upon theworkpiece as the workpiece is traversed before it, or as the beam istraversed across it, and simultaneously creates a certain level of heatand accompanying IR radiation, the signals of which are detectable byand picked up by an IR detector means. The IR radiation is the productof temperature and emissivity. For certain products, depending upontheir character, it may be assumed that the emissivity thereof willremain generally the same, in which event then any detectable changesare due to temperature changes in the surface.

A known type of IR detector means 90 is appropriately supported closelyadjacent to the focused beam spot or point P'. Because IR detectors havesome sensitivity to visible light, it may be necessary that a shieldmeans 92 be suitably interposed between the beam and the IR detector 90to screen out the beam's intense visible light. For thin specimens thedetector means 90 may be disposed to view the back of the specimen orworkpiece instead of the front. Because everything inherently emits someIR radiation, the detector 90 is adapted to detect and indirectlymeasure the temperature of the focused spot or point P' throughattendant IR radiation or emission when the emissivity factor is knownof the material being treated. Thus, the detected IR signal is fedthrough a first conductor cable means 48c, via an optional separatepreamplifier means 50c, into one input side of the lock-in amplifiermeans 54c. In the manner generally described relative to FIG. 2, similartype optical emitter-detector means 70c, 72c are used therewith togenerate a reference input signal via further cable means 78c into thelock-in amplifier means 54c. The two input signals are appropriatelyprocessed internally in a known manner and a D.C. voltage output signalis feedback via conductor cable means 84c to an appropriate feed backcircuit means 86c, then via conductor 88c to control means on a suitablycorresponding beam generator means (not shown). In this manner theappropriate control is achieved and maintained during the laser treatingprocess. In the gas cell photoacoustic detection approach on solids, thegas cell volume should be minimized for a better signal to noise ratio.This consideration leads to small gas cell detectors so the IR techniquehas an advantage relating to the treatment of larger size work pieces.

The use of the IR detector mode or technique is such that it can becarried out equally well under normal atmospheric conditions, and alsowithin a vacuum. The broken box-like outline denoted 94 in FIG. 3 isdiagrammatically representative of housing means which can be evacuatedto enable the process to be carried out within vacuum conditions.However, this would also necessarily limit or restrict access to theobject being processed.

Although a chopper disc means has been shown and described as anexemplary means for effecting beam modulation, it is understood that analternate mechanical (chopping) means may include vibrating reed meansif chosen to have the correct frequency of vibration. It is preferablethat such a reed would vibrate at a single frequency like a tuning fork.Still other means for effecting beam modulation may includeacousto-optic devices, one form as known commercially being called anAcoustic Optic Modulator, such as manufactured by Intra Action Corp., ofBellewood, Ill. 60104; or some still further different form of directcontrol of the beam.

Reference to FIG. 4 and to an exemplary "Mirage" effect technique orsystem will now be described. A similar high energy main beam 14d, suchas a laser beam, is similarly focused into a point P" upon a CVD coatedor other suitably coated workpiece 16d. Beam 14d is modulated by anysuitable means, such as chopper disc means 58d. The focused main beam14d generates a heated area or hot spot in the area of P", and in sodoing heats up the air above the surface of the solid workpiece 16d. Thearea of heated air is designated by the exaggerated bump or dash rippleline 96. A further suitable beam generating means is used to generate aprobe beam for merely monitoring and detecting the selectedphotoacoustic effect set by beam 14d. Such a probe beam does not exciteor generate a second photoacoustic effect. Thus, for this exemplaryembodiment a second laser generator means, shown fragmentarily at 13,generates a second laser beam 15 which is used as a probe beam. Beam 15may be of the Helium-Neon type, and is directed so as to pass closelyabove the surface of the workpiece 16d while intersecting the beam 14djust above the hot spot P". Upon its passing through the heated area 96,with the attendant changed lower index of refraction, the beam 15 isslightly deflected upwardly as shown at 15a in FIG. 4. If the beam 15 isslightly off the center of excitation beam 14d, it will also bedeflected to the side as well as upwardly. This can be used to providesome additional information about the heated spot. Deflected beam 15a isintercepted by a strategically placed detector means 98 aptly supportedbeyond the workpiece 16d. It is preferred that during the producttreating process or other process to be controlled, as long as thetemperature of the surface area being monitored is detected as beingwithin a predetermined acceptable temperature range, then there is noneed to alter the power setting of laser generating means 13. Thus, thedetector means placement is such that during the aforementionedfavorable operable time periods, only a predetermined insignificantpercentage of the deflected probe beam directly impacts upon a potentialsignal-generating cell portion of the said detector means 98.

Various suitable auxiliary means may be used to assure that atpredetermined times or for a given operational condition, only a certainpredetermined percentage of the beam will be cast upon the detector. Onesuch auxiliary means, to assure the foregoing condition, is to use astrategically placed slotted barrier means 99, or at least a partialbarrier, which would intercept a predeterminable portion of the beamunder certain circumstances. More preferably, the barrier means 99 wouldbe of adjustable character, not only to vary the slot opening but therespective heights of respective upper and lower half portions thereof.The detectors may vary in area size to meet varying specific needs. Thedetectors also may be wired into circuits of different operativecharacter. That is, if an acceptable temperature range is beingmaintained for the part being monitored or treated, and the probe beamis so positioned that a predetermined limited insignificant portion ofthe deflected beam, as controlled by the barrier means 99, is impactingupon the potential signal-generating detector means, then the controlsettings of the main excitation beam are left in status quo until anunsatisfactory imbalance occurs. In the latter event and upon the probebeam becoming deflected to a greater extent so as to impact moresignificantly upon the detector means, the detector transforms thechanging optical signal to an electrical signal of usually greaterpredetermined magnitude and/or character to effectively cause thefeedback circuit means to take appropriate corrective action of the beamcontrol means. Alternatively, the circuitry could be designed for anopposite arrangement, i.e. as when the probe beam is hitting more fullyand directly upon the detector means for a given satisfactoryperformance setting, it effects a stable control of the beam generatormeans until a different unsatisfactory imbalance might occur, as by lossof beam strength hitting the detector. If this latter condition occurs,then a different preprogrammed signal is emitted to effect thepredetermined desired corrective control.

The detector means may comprise a photocell type, a silicon photodetector, a cadmium sulfide or similar light-responsive type detectorwhich is capable of transforming the light beam signal into anelectrical signal.

A more specific exemplary description will now be reviewed for theaforedescribed system and process, assuring that proper controls can beadministered when and if the monitored character of the surfaces undertreatment of the basic beam 14d is detected as changing from the desirednorm. For example, if the spot P" begins to overheat, increasing theambient temperature in area 96 beyond the acceptable range, then beam15a is deflected to a sufficiently greater extent as to more fully coverthe detector 98, and cause the signal-generating cell portion to emit astronger signal. This signal is fed into the lock-in amplifier 54d, viaa preamplifier means 50d, if needed, and the related conductor cable48d. A similar reference input signal is generated by emitter-detectormeans 70d and 72d of the main excitation beam's modulating means in thesame aforedescribed general manner. This signal is similarly carried viaconductor cable 78d into the lock-in amplifier means 54d. These signalsare also then processed internally by the lock-in amplifier means 54d toproduce a D.C. voltage output signal which is conducted via cable means84d and via feedback circuit means 86d to the control means on the beamgenerator, neither of which are shown in this FIG. 4 embodiment.

As previously indicated, the photoacoustic effect is considered togenerate electrical signals with values representative respectively ofboth the magnitude and phase thereof. Under certain circumstances, itmay be that either only the magnitude part of the signal or only thephase part of the signal will be utilized by the signal processing meansfor maintaining and/or modifying as needed the beam-generating powercontrol means of a processing system. Under other circumstances, boththe magnitude and phase representative signals will be used collectivelyfor maintaining and/or modifying the beam-generating power controlmeans, as needed.

In any of the arrangements described herein, or in equivalentarrangements, where the generated signal may prove to be strong enough,compared to background noise, then use of either the preamplifier and/orlock-in amplifier means may not be necessary. In such event, the signalis simply rectified and applied directly to the applicable controlmeans.

Although the illustrative FIG. 4 embodiment depicts only a singledetector means 98 in conjunction with the so-called "Mirage" mode ofdetecting an acoustical effect, it is to be understood that anappropriate multi-detector array means, together with the appropriateelectronics, may be desired for some embodiments to detect variousdegrees of deflected beam positions and to generate the appropriatecorrective signal. Separate control means are needed to respectivelyincrease the power setting as well as to decrease the power setting,depending upon the needs of the particular system or process beingcontrolled. In instances where a pulse type laser is used, appropriatecontrol circuitry will be used so that the produced DC voltage signalfrom the lock-in amplifier will be useful to control the lasergenerator's pulse power, pulse width, or repetition rate, within limits.

FIG. 5 is a diagrammatic, fragmentary representation of a process usingion beam implantation. In association therewith is a contemplatedpiezoelectric type detecting means to effect desired monitoring andcontrol of the ion beam, which will now be described. Ion beam generatormeans 100 generates within an enclosed vacuum area (not shown) an ionbeam 102 which is aimed toward a subject workpiece 104 to be implantedwith ions. Workpiece 104 has attached thereto detector means 106 in theform of a piezoelectric transducer. Although transducer 106 is shown onthe underside of workpiece 104, it is understood that it may bealternatively positioned on an end or top side as indicated in brokenoutline at 106'. It is preferable that the transducer be applied to aside of the workpiece in a manner which avoids application of anycompressive force to the transducer. In this form, an electric field108, established by a laterally applied positive voltage source, from asuitable power generator 109 is used to modulate beam 102 byelectrostatically deflecting or steering the ion beam as at 102a backand forth across the slot 110 of a slotted modulating barrier 111. It isunderstood that if the voltage applied is positive, the beam will bedeflected away in a given direction, and if negative it will deflect inthe other direction. It is further understood that the main excitationbeam may be other than an ion beam, including electron beams, particlebeams, neutron beams, and intense beam forms, such as an ultravioletbeam, coming within the electromagnetic spectrum, along with associatedappropriate modulating means.

Referring further to FIG. 5, responsive to the bombarding of ionsagainst the surface of the workpiece or substrate 104, a modifiedsurface 104a denoted in an exaggerated manner by the dashed line 104a,evolves as the result of ion implantation. The modulated impact of theions induces repetitive heating and cooling which, in turn, causesrepetitive expansion and contraction. This repetitive expansion andcontraction initiates a vibration effect accompanied by the inducedmechanical waves depicted at 112. Thus, the induced mechanical wavesexcite the piezoelectric detector 106 producing an electrical signal.

A modulating reference signal from generator 109 is transmitted viaconductor 109a into the lock-in amplifier 54e. The other input signalinduced by the thermoelastic properties of the material via thetransducer 106 is then conducted via conductor line 48e into the lock-inamplifier 54e, in the same general manner as described relative to FIGS.3 and 4. An optional preamplifier means denoted in broken lines at 50emay be incorporated into the circuitry, if needed. After the two inputsignals are processed within the lock-in amplifier 54e, an output D.C.voltage signal is conducted via conductor line 84e, and thence viaappropriate feedback circuit means 86e, to a corresponding ion beamcontrol means (not shown). From this exemplary arrangement, it isequally apparent that use of this photoacoustic effect (orphoto-calorimetry effect) in conjunction with utilizing piezoelectrictransducer detected vibrations of the material being treated orprocessed, does provide a feasible and unique manner of controlling ionbeam processing. This is achieved in a similar manner as used to controlthe laser beam processing examples of FIGS. 2, 3, and 4.

Before proceeding with still a further alternative detecting techniquerelative to FIG. 6, it is to be understood that a process control systemor method may embody at least two separate photoacoustic detectingtechniques which may be respectively used to function in an individualmanner yet collectively with the apparatus or system performing theoverall control method. An example of this can be seen by againreferring to FIG. 2 which had basically shown the gas cell means andtechnique. For exemplary purposes, a second detection technique andmeans is shown only by the addition of another piezoelectric transducermeans 107 therein, shown attached to the left-hand side of the workpiece16'.

Reference now will be made to the still further "Reflection" mode ortechnique, as depicted schematically in FIG. 6. In FIG. 6, theprocessing laser beam is denoted 14f which is focused to a smalltreating spot or point P'" upon the CVD coated substrate or other typeworkpiece 16f undergoing a treatment process. The beam 14f may bedisposed with its center axis "a" generally perpendicular to portion ofthe workpiece being treated. Beam 14f is suitably modulated, as by thechopper disc means 58f; in conjunction therewith is an appropriateoptical emitter-detector means, which may be of the type shown in anddescribed relative to FIG. 2 and 4. The optical emitter-detector means,designated 70f,72f, is wired into the circuit means so that thegenerated optical signal is converted into an appropriate electricalsignal. This latter reference signal is fed via conductor cable 78f intothe left side of lock-in amplifier means 54f. When the surface ofworkpiece 16f becomes heated, the heated spot expands and bulges outslightly as a manifestation of the thermo-elastic effect. This is shownby the greatly exaggerated bump line 97. Also the air above the spot isheated as usual in the photoacoustic effect forming the area of heatedair designated by the dash ripple line 96'. In this embodiment, theincoming portion of the probe beam is denoted 115, and is aimed at theheated area P'" at a preselected angle which may be determined at leastin part by the contour of the workpiece being treated. In theillustrative example shown in FIG. 6, the workpiece 16f may be a turbineblade or other irregularly curved or shaped component, where practicalapplication of the "Mirage" technique would be substantially precludedon its concave surface. Therefore, use of this so-called "Reflection"technique may be preferred. In this arrangement, the normal line denoted"a" which is normal to the surface at the point of impact of theincoming beam 115, the incoming beam 115, and reflected beam 115' alllie in a common plane. As shown, under the laws of reflection the angleθ of the incoming beam relative to the said normal line "a'" is equal tothe angle θ' of the reflected beam 115' relative to the same normal. Thereflected beam is deflected by both the change in surface contour andthe thermal lensing utilized in the "Mirage" technique. It is possiblethat other processes are also contributing to the deflection in thisnewly reported technique. The incoming beam 115 is reflected off of theexpanded heated areas 96' and 97 as shown, and is picked up bystrategically placed detector means 98'. Thus, by monitoring thedeviation of the probe beam reflection 115' from the hot spot, anappropriate correlation can be made between the detected deviation andthe degree of heat being applied by the processing beam so that anappropriate range of signals is generatable, and is accordinglygenerated by the transducer of detector means 98' for input viaconductor cable 48f, and the optional preamplifier means 50f into thelock-in amplifier means 54f. These two input signals are then processedby the lock-in amplifier means to produce a meaningful output signal, inthe manner described relative to the previous embodiments. Thus, theoutput signal is transmitted thru an appropriate feedback circuit means86f which in turn effectively controls the processing beam 14f.

The "Reflective" technique needs a moderately reflective surface, andthus would not be as appropriate for use on very absorbent or very roughsurfaces or surfaces having a very low coefficient of expansion. This"Reflective" mode is considered to be potentially more sensitive withregard to strength of signal to background noise, than obtainable withsome of the other techniques.

From the foregoing descriptions, a summary now follows of some of therelative advantages and disadvantages for the several described modes ortechniques of detecting the photoacoustic effect.

PHOTOACOUSTIC (EFFECT) DETECTION TECHNIQUES Gas Cell

Advantages: Easy to apply to a regular surface; most developedtechnique.

Disadvantages: Requires coupling gas. Requires cell of relatively smallsize and suitable shape to fit part, thus making it somewhat moredifficult to implement on large parts or parts of irregular shape.

Mirage

Advantages: Where imperfection cracks appear in a material being treatedor monitored, this technique results in better crack signatures ordefinition; lower modulation frequencies are typical allowing deeperprofiling.

Disadvantages: Not appropriate for concave surfaces; not appropriate foruse in a vacuum. Requires careful alignment of the parts surface, theprobe beam, and the focal spot of the excitation beam; provides slowerscans than available with the gas cell technique.

Piezoelectric

Advantages: Will work on most component shapes; will work in a vacuum.Could be used to gain information on the interior of the part that themechanical wave traverses. Fast scan rates are possible.

Disadvantages: Poor where material has low coefficient of thermalexpansion or absorbs the mechanical vibrations. Requires good acousticalmatching to the sample. Could be misleading if mechanical resonances areexcited in the part. The conversion efficiency of thermal waves intoacoustical waves is very low.

Infrared (IR)

Advantages: Works well in air and in a vacuum.

Disadvantages: Signal generated is the product of both emissivity andtemperature, rather than only temperature.

Reflection

Advantages: Good for use on surfaces of irregular shape; shows promiseof a relatively strong signal. Fast scan rates may be possible.

Disadvantages: Not very appropriate for absorbing surfaces, or for roughsurfaces; i.e. requires surfaces with a moderate reflectivity. Morecomplex equipment and alignment necessary.

CONCLUSION

From all the foregoing description, it is apparent that the objectivesand the advantages of this application have been achieved, by the uniqueprocesses hereof in conjunction with the variously described indirectmodes of monitoring and using temperature variations of the workingportions of the workpieces.

Although the invention has been described and illustrated with respectto several embodiments, it is readily apparent that those skilled in theart will be able to make still other modifications of the exemplary oneswithout departing from the spirit and scope of the invention as definedin the appended claims.

What is claimed is:
 1. A novel method for controlling various processesby utilizing a photoacoustic effect principle, whether or not thevarious processes are with regard to a solid or non-solid fluid medium,said processes using and requiring control of a primary excitation beamof the type including laser beams, microwave beams, x-rays and otherelectromagnetic wave beams, as well as ion, electron, particle andmolecular beams, and which processes include the use of appropriatebeam-generating means and beam-generating power control means inconjunction therewith, said novel method comprising the steps of:(a)generating and modulating in a pulsing manner a selected type of primaryexcitation beam; (b) directing said beam at a workpiece or work area tobe treated thereby, so as to create a temperature variation thereat, andso as to establish a photoacoustic effect; (c) continuously monitoringthe beam-impacted work area by one of several different photoacousticeffect detection techniques which includes means for detecting thecharacteristic temperature variation in said area, which variation canmanifest itself as pressure, index of refraction, thermoelastic, orinfrared (IR) radiation changes, considered singly and in variouscombinations; (d) said monitoring step resulting in generating anelectrical signal by appropriate transducer means, with the signal to beused by ancillary signal processing means; (e) said monitoring furtherusing the selected type photoacoustic effect detecting means fordetecting any such work area temperature variation outside of apredetermined acceptable - unacceptable temperature range which isindicative that the given process with which this control procedure orprocess is being used is in need of correction, and (f) using saidelectrical signal as generated in conjunction with said monitoring meansof sub-paragraphs (c) (d) (e), together with ancillary signal-processingcontrol circuit means for effectively maintaining and modifying asneeded the beam-generating power control to assure proper overallprocessing.
 2. The method of claim 1, wherein said monitoring anddetecting includes utilizing a gas cell technique utilizing related gascell means for detecting the photoacoustic effect and an attendantcharacteristic temperature variation of the treated work area.
 3. Themethod of claim 1, wherein said monitoring and detecting includesutilizing a "mirage" type photoacoustic detecting technique togetherwith "mirage" - generating probe-beam means and attendant photodetectiontransducing means for detecting the attendant characteristic temperaturevariation of the treated work area.
 4. The method of claim 1, whichincludes using a piezoelectric detector type technique by usingpiezoelectric detector-transducer means for detecting the characteristictemperature variation of the treated work area.
 5. The method of claim1, which includes using an infrared (IR) detector type techniquetogether with infrared (IR) detector means for detecting the attendantcharacteristic temperature variation of the treated work area.
 6. Themethod of claim 1, wherein said monitoring and detecting includesutilizing a reflection type photoacoustic detecting technique togetherwith reflection generating probe beam means and attendantphoto-detection transducing means for thereby detecting the attendantcharacteristic temperature variation of the treated work area.
 7. Themethod of claim 1, wherein the photoacoustic effect includeselectrically generated signals representative respectively of both themagnitude and phase thereof, but using only the magnitude part of thesignal for maintaining and modifying as needed the beam-generating powercontrol means.
 8. The method of claim 1, wherein the photoacousticeffect includes electrically generated signals representativerespectively of both the magnitude and phase thereof, but using only thephase part of the signal for maintaining and modifying as needed thebeam-generating power control means.
 9. The method of claim 1, whereinthe photoacoustic effect includes electrically generated signalsrepresentative respectively of both the magnitude and phase thereof, andusing both the magnitude and phase representative signals formaintaining and modifying as needed the beam-generating power controlmeans.
 10. The method of claim 1, wherein at least two separatephotoacoustic detecting techniques are used individually to determinethe photoacoustic effect in their respective ways to therebycollectively effect the beam-generating power control of sub-paragraph(f).
 11. The method of claim 1, which includes processing the electricalsignal of sub-paragraph (d) by means of a lock-in amplifier means whichuses as its reference a further signal generated in conjunction with abeam-modulating means, said further signal being representative of thefrequency of the beam modulation; and, which processing is done toseparate the photoacoustic signal from various interfering signals sothat it can be effectively used by said ancillary control circuit meansof sub-paragraph (f).
 12. Apparatus for controlling various differentprocesses, whether with regard to a solid or non-solid fluid medium,using any one of various primary excitation beams including laser beams,microwave beams, x-rays and other electromagnetic wave beams, as well asion, electron, particle, and molecular beams, said apparatusincluding:(a) beam generating means and beam-generating power controlmeans operatively connected therewith and with a power source; (b) meansfor modulating said beam in a pulsing manner; (c) means for directingthe beam to and thereby heating of the area under treatment therebycreating temperature variation in the workpiece or work area; (d) meansfor monitoring the beam-impacted work area, said monitoring meansutilizing one of several different photoacoustic effect detectiontechniques and related means for achieving it, for detecting thecharacteristic temperature variations in the workpiece or otherbeam-directed work area, which temperature variations can manifestthemselves as pressure, index of refraction, thermo-elastic, or infrared(IR) radiation changes, considered singly and in various combinations;(e) transducer means in conjunction with and constituting part of saidmonitoring means to convert a detected temperature-variation-inducedmanifestation of the photoacoustic effect into an electrical signal forinput to and usage with signal processing means; (f) said means formonitoring of sub-paragraph (d) also including means for detecting anysuch work variations of paragraph (d) which are beyond a predeterminableacceptable-unacceptable temperature range which is indicative that thegiven process with which this control apparatus is being used is in needof correction; and (g) control circuit means including signal processingmeans which are collectively operable to effectively maintain and modifyas needed the beam-generating power control means to assure the properoverall processing.
 13. The apparatus of claim 12, wherein said means ofsub-paragraph (b) for modulating the beam include mechanical choppingmeans.
 14. The apparatus of claim 12, wherein said means ofsub-paragraph (b) for modulating the beam include acousto-optic means.15. The apparatus of claim 12, wherein said means of sub-paragraph (b)for modulating the beam include direct electrical control means.
 16. Theapparatus of claim 12, wherein the photoacoustic detecting means ofsub-paragraph (d) includes photoacoustic gas cell means.
 17. Theapparatus of claim 12, wherein the photoacoustic detecting means ofsub-paragraph (d) is of the "mirage" type which includes attendant probebeam mean including a generated probe beam, and related photo detectiontransducing means, said probe beam disposed to pass closely adjacent andgenerally parallel to the beam-impacted area of said primary excitationbeam.
 18. The apparatus of claim 12, wherein said photoacousticdetecting means of sub-paragraph (d) is of the piezoelectric transducertype and includes piezoelectric transducer means.
 19. The apparatus ofclaim 12, wherein said monitoring and detecting means of sub-paragraph(d) include infrared radiation detector means.
 20. The apparatus ofclaim 12, wherein the photoacoustic detecting means of sub-paragraph (d)is of the reflective type which comprises reflecting probe beam means,and a related transducer means strategically disposed to intercept andreceive the reflected probe beam, said latter-mentioned relatedtransducer means constituting the said transducer means of sub-paragraph(e).
 21. The apparatus of claim 12, further including lens means inconjunction with certain apparatus arrangements, said lens meansadaptable for focusing the generated primary excitation beam into arelatively small spot area or point to be directed to a work area aspart of a process being controlled by this apparatus.
 22. The apparatusof claim 12, further including means for effecting relative movementbetween said generated beam and a workpiece or work area undergoingprocessing or treatment by said primary excitation beam.
 23. Theapparatus of claim 12, wherein said control circuit means ofsub-paragraph (g) further includes lock-in amplifier means as part ofthe signal processing means, and within which lock-in amplifier meansthe generated signal of sub-paragraph (e) is processed together with areference signal generated in conjunction with said modulating means ofsub-paragraph (b), to thus separate the photoacoustic signal from anyinterferring background signals to facilitate more effective use of thephotoacoustic signal as part of said control circuit means.
 24. A methodfor optimizing uniformity of treatment of the surface of a solid workpiece having non-uniform optical and thermal absorption properties on ornear the surface with a variable intensity primary excitation beam, saidmethod utilizing a photoacoustic effect principal, a power control, anda photoacoustic effect detector; said beam intensity being controlled bysaid power control; said method being primarily directed to maintainoptimum beam intensity on the surface being treated by varying said beamintensity as a function of the physical properties of a treatment area;said method comprising the steps of:a. imaging said beam on saidtreatment area to raise the temperature of the surface; b. modulatingsaid beam in a pulsing manner to induce a photoacoustic effect having amagnitude and phase at said treatment area; c. detecting saidphotoacoustic effect produced by said pulsing beam with saidphotoacoustic detector means; said detector means generating anelectrical signal output corresponding to said magnitude and phase ofsaid photoacoustic effect, which output is proportional to thetemperature at the treatment surface; d. processing said output toconform the signal to a condition acceptable by said power control; e.directing said output to feedback to said power control; f. adjustingthe power control in response to the feedback, thereby varying said beamintensity to maintain said optimum beam intensity on said treatmentarea; whereby said work piece receives a substantially uniform treatmentrelatively independent of non-uniform optical and thermal absorptionproperties on or near the work piece surface.
 25. The method of claim 24wherein said primary excitation beam is of the type including laserbeams, microwave beams, x-rays and other electromagnetic wave beams. 26.The method of claim 25 wherein said photoacoustic detector means is ofthe type including gas cell, "mirage," piezoelectric, infrared andreflection.